Optimisation of cushion materials for rockfall protection galleries

Rockfalls of various magnitudes occur regularly in mountainous regions and Switzerland is no exception. Protection galleries made either of reinforced concrete or pre-stressed concrete members, with a cushion material laid on top, could be used more efficiently to prevent rockfalls that lie within the prevalent range from damaging infrastructure and lifelines. This requires greatest care in developing the general concept of design and construction, considering the ductility requirements of the structure and the selection of material for the energy absorbing cushion layer. Currently, design of the gallery in Switzerland is dependent upon the impact force selected as the "action" and the corresponding penetration depth of the falling rock into the cushion material with design limit of 3 MJ. Main aim of this research is to increase the efficiency of energy absorbing capacity of the rock protection galleries using standard and new cushion materials while improving the existing design methods. This work on cushion materials for rockfall protection galleries is divided into experimental, numerical and analytical modelling of a rockfall event at prototype energies. Prototype energy scenarios are simulated in the centrifuge using small-scale models, which are exposed to enhanced gravity to replicate a full-scale event, by taking advantage of wellproven scaling laws. Impact energy conditions are varied (up to 20 MJ) and detailed impact force and stress distributions on top of a gallery are measured using state-of-the-art measuring units under defined boundary conditions. A systematic variation in the system parameters such as mass, fall height, impact angle of the rock boulder, type and thickness of the cushion material and finally the type of construction of the cushion material are carried out. New ideas for cushioning materials for damping the impact on top of the galleries are also investigated. Laboratory investigations supplement the modelling, to determine the damping parameters of cushioning materials as well as the integrity and response of the boulder and the associated parameters. The physical modelling is also supported by numerical modelling on the overall behaviour of the system, and the local behaviour at the point of impact. Finite element modelling using LS-DYNA is carried out at prototype stress levels varying key parameters mentioned above influencing and determining the stresses induced in the gallery. An analytical model using lumped parameters such as spring, dashpot and slider is presented to model the cushion material behaviour under rock boulder impact and to determine the impact force acting on the gallery and the penetration of the rock boulder into the cushion material. Based on the possibility of using an easy calculation method, preliminary guidelines have been presented for a practising engineer. These include the design of new and standard cushion materials as well as the recommendations for different scenarios possible. This research can be applied in recalibration of the existing galleries and for the dimensioning of the new protection galleries.